1. Field of the Invention
The present invention relates to a focus control method for Delta-Sigma based image formation devices, in particular, to a newly developed focus control method by delay-sum calculation for an image formation device capable of eliminating noises when a Delta-Sigma based image formation device is performing dynamic focusing without affecting advantages of the single bit output of the Delta-Sigma converter.
2. Description of the Prior Art
Digital image formation has been widely applied in medical ultrasound because it is capable of providing precise focusing. In a conventional digital ultrasonic imaging system, 8-10 bit analog/digital (A/D) converters are used with a sampling frequency from 20˜40 MHz. At this frequency, it is necessary to employ an interpolation circuit to achieve an effective time delay accuracy at 32 times the center frequency. Conventionally, filtering or phase rotation is employed to up-sample accuracy of delay value. On the other hand, a Delta-Sigma based analog/digital converter {DS A/D} was suggested to design a ultrasound imaging system. In this case, the complicated interpolation circuit is no more required because a higher sampling frequency has been obtained, a single bit output can greatly reduce system size, and a system utilizing the DS A/D converter can obtain the same image quality as that of a conventional system while the system complexity is greatly simplified.
The advancement of VLSI has increased the speed of the digital circuit more than the improvement of the accuracy of an analog circuit. Hence, it is preferable to use an over-sampling A/D converter and subsequent digital signal processing. Due to the difficulties concerning circuit design, such an A/D converter has been used for signals at lower frequencies. Since each input signal affects the output signal, a filter is required to reconstruct the original signal. Referring to FIG. 1(a), wherein x[n] is an output, and y[n] represents an output before reconstruction. By using a noise to model the quantization effect as shown in FIG. 1(b), wherein e[n] represents the inserted quantization noise. In discrete time, the following equation is obtained:Y(Z)=X(Z)Hx(Z)+E(Z)He(Z)  (1) wherein Y(Z) is an output signal, X(Z) is an input signal, E(Z) is the added quantization noise, while Hx(Z) is a transfer function of the input signal and He(Z) is a transfer function of the noise. The aim of oversampling is for obtaining a sufficient bandwidth for the noise energy.
For a Delta-Sigma converter, signal to quantization noise ratio (SQNR) after signal reconstruction can be expressed as:SQNR=A×log2OSR+6×(B−1)+C(dB)  (2) Wherein A is a constant in relation with the order of the A/D converter and noise shaping. OSR is an over sampling ratio which is defined as a ratio of sampling frequency to Nyquist frequency wherein Nyquist frequency of a typical ultrasound signal is generally 3˜4 times of the center frequency. B is the number of bits of a quantizer which is generally 1 for a Delta-Sigma converter, C is a constant determined by the dynamic range of the signal. Among them the most critical factors are A and OSR. As an example. 40 dB of SQNR, which is approximately equivalent to a convenient 7-bit A/D converter, can be obtained by using second-order low pass Delta-Sigma converter with an 8 times over sampling ratio. A further improvement is possible by employing a higher over sampling ratio.
In a scheme shown in FIG. 2 which utilizes a two-order low pass Delta-Sigma converter, x[n], e[n], and x*[n] respectively represent an input, an added quantization noise, and an output, all working at 32 times the signal center frequency. The Delta-Sigma converter increases the SQNR due to the over sampling frequency. In this scheme, as the sampling frequency is doubled, SQNR will be raised by 15 dB, equivalent to 2.5 bits for conventional A/D converters. For each channel, the input signal x[n] is converted to a single bit output y[n] by the Delta-Sigma converter. The signal after sampling is stored in a single bit shift register, and then is appropriately delayed by a controller. As the sampling frequency is 32 times the signal center frequency, the interpolation circuit is not required. The single bit signal obtained from each channel are delayed and summed to form a beam. The quantization noise in present in the high frequency region and it can be filtered out by a low pass filter, also known as the reconstruction filter. After reconstruction, the signal is decimated to 4 times of the center frequency so as to reduce high speed computations. The entire image formation system is shown in FIG. 3, it is an image formation device designed by using Delta-Sigma converters. After the signal is time gain compensation (TGC), it is converted to a single bit signal through Delta-Sigma A/D converter. This signal is then stored in a shift registor to be selected by the delay controller and a multiplexer. The delayed signal from each channel on the probe is summed to obtain a beam which further goes through the low pass filter for reconstruction and is decimated to a lower frequency so as to simplify subsequent signal processing. On the other hand, TGC is also used to match the dynamic range of the Delta-Sigma converter.
The system in this scheme has a new problem when carrying out the dynamic delay control. Due to the fact that synchronization between modulation and demodulation is destroyed by dynamic focusing, the signal energy is affected and the background noise increases. Several solutions have been proposed, but all these require extra bits to encode the output, resulting in increased system complexity.
It is what the object that the inventor has endeavored for many years in conducting intensive research and simulations in order to solve the above depicted shortcomings of the conventional system, and finally succeeded in realization of the present invention.